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Antivirus and e-mail security companies sent out warnings on Tuesday about a new Trojan horse program that they claim is being mass distributed on the Internet using unsolicited commercial, or spam, e-mail.

The program, called Backdoor-CGT, is a new form of a Trojan horse installed after e-mail recipients using Microsoft Corp.’s Outlook e-mail program follow a Web link embedded in an e-mail message. The Trojan horse is believed to have infected thousands of systems on the Internet since appearing early Tuesday, even though antivirus software and up-to-date versions of Outlook are immune to attack, according to Maksym Schipka, senior antivirus researcher at MessageLabs Ltd. in the U.K.

MessageLabs received more than 3,600 e-mail messages with links to the Trojan horse during a two-hour period early Tuesday, the result of a massive and uncharacteristic spam distribution more than 10 times what is normal for such a program, he said. Trojan horse programs give remote attackers access to or control over machines on which they run, and often run unnoticed by computer users, or pose as legitimate software applications.

The Backdoor-CGT Trojan uses a “multistage” attack to place malicious code on victims’ computers. After clicking on an e-mail link embedded in the spam message, victims go to a series of Web sites, each of which carries out one stage in the attack. The attack takes advantage of a now-patched flaw in Outlook called the “IFRAME” exploit to hide the Web site redirections from the user and silently download and install the Backdoor-CGT program, Schipka said.

Once installed, Backdoor-CGT selects a communications port at random and opens it, creating a back door on infected systems that is used to communicate with a server on the Internet supposedly controlled by those behind the attacks. The Web site used by the compromised machines is registered in the .biz Web domain to an individual in the Czech Republic and was still online, though slowed by heavy traffic, on Tuesday, he said.

McAfee Inc. also released an advisory about the new Trojan program, also known as “SS,” on Tuesday, but rated it “low,” indicating it does not pose a great threat to either home or business users.

Other antivirus companies did not immediately respond to requests for information about Backdoor-CGT and it was not clear whether other companies were aware of it, or whether other antivirus software programs could spot the new malicious program.

However, before the Trojan program can be downloaded and installed, the attackers attempt to place a common version of another program, called a “dropper,” that antivirus programs can spot, thwarting infections, Schipka said.

Microsoft Outlook users are advised to apply the latest software patch for the product to prevent infection, he said.

McAfee released software update files to detect the new Trojan program on Tuesday, according to the company advisory.